In the present technical field, in addition to requirements of high speed and high quality communications, it is also necessary to meet the needs of increasing the number of users accommodated in the system, and it is important to efficiently utilize communication resources such as frequencies.
A multi-carrier transmission method, which has attracted attention in recent years, uses plural sub-carriers to transmit data, thereby, for example, resulting in high transmission speed, and improving resistance to frequency selective fading. However, since the multi-carrier transmission method uses plural sub-carriers having various frequencies from low to high, depending on the communication environment, the signal quality may be degraded because of frequency offsets such as Doppler shifts.
On the other hand, in the multi-carrier transmission method, it is attempted to improve signal quality by using an adaptive array antenna (AAA). Specifically, weighting factors in the array are adaptively adjusted so that the magnitude of a signal component associated with a virtual sub-carrier in the received signals becomes zero, thereby reducing waves influenced by the Doppler shift and improving quality of communication signals. (For this technique, for example, reference can be made to H. Hanegi et al., “OFDM Null Steering Array Antenna”, Transactions of the Institute of Electronics, Information and Communication Engineers, 2002-124, 2002, July). Generally, methods of the adaptive control include beam forming and null steering, wherein the former controls the main lobe to point to the desired wave, and the latter controls so as to suppress the undesired wave to null. Here, the latter one is used.
The virtual sub-carriers are sub-carriers not used for transmitting data, among the sub-carriers included in the frequency band assigned to the system, while the sub-carriers used for transmitting data are referred to as “data sub-carriers”. It is fixed by the system which sub-carriers among many sub-carriers are the virtual sub-carriers. For example, as illustrated in the upper portion of FIG. 1, in order to reduce out-of-band emission power in non-linear amplification, some sub-carriers at the ends of the frequency band assigned to the system are set to be the virtual sub-carriers. (For this technique, for example, reference can be made to ARIB STD-B24, “Data Coding and Transmission Specification For Digital Broadcasting”, ARIB, June, 2000). In addition, as illustrated in the lower portion of FIG. 1, in order to reduce direct current drift during base band processing of the received signal, some sub-carriers near the center of the frequency band are set to be the virtual sub-carriers. (For this technique, for example, reference can be made to ARIB STD-T70, “Lower Power Data Communication Systems Broadband Mobile Access Communication System (CSMA)”, ARIB, December, 2000).
Specifically, among 64 sub-carriers, 52 sub-carriers are set to be data sub-carriers, and 12 sub-carriers are set to be virtual sub-carriers. Because the virtual sub-carriers fixed in this way are not used for data transmission, in modulation during signal transmission, the signal components associated with the virtual sub-carriers are set to be zero.
FIG. 2 is a diagram schematically illustrating a modulator in OFDM (Orthogonal Frequency Division Multiplexing). In OFDM, signal modulation is performed by the Inverse Fast Fourier Transformation (IFFT). For this reason, the time-series transmission data shown on the left side in FIG. 2 are converted into parallel signals by a series-parallel converter (S/P), and are input to an IFFT unit. All contents included in the parallel signals are in correspondence to the data sub-carriers. Contents of signals associated with the virtual sub-carriers are permanently fixed to be zero. Based on signals input and designated in such a way, the Inverse Fast Fourier Transformation is performed, modulated parallel signals are output, and afterward, through processing necessary for radio transmission, radio transmission is executed.
However, because the above techniques in the related art are not intended for distinguishing radio signals, it is difficult to use frequency resources simultaneously even with the above techniques. For example, when a user A is performing radio communications, another user B cannot perform radio communications. Even when the user A and user B are at geometrically different locations, one of them has to wait until communication (time slot) of the other one is finished.